Method of making a stable electricallyconductive sheet and product thereof



Sept. 19, 1%?

02/50 600 TIA/6 mm ee squnee T. J FITZPATRICK 3,341,942 METHOD OF MAKING A STABLE ELECTHICALLY-CONDUCTIVE SHEET AND PRODUCT THEREOF Filed Dec. 24, 1964 MIL W5 7' CORT/1V6 MICK/V553 7 MIL W57 60077416 77/ a 700 00 906 moo 000 A200 /aoo GEQMS GE/JPH/ A0060 To ism/009120 L/QU/D CONTQ/IV/NG Z4 00 699 43 OFH-5-Z1JDOX 3 341,942 METHOD F MAKWG A STABLE ELECTRICALLY- CgNDUCTIVE SHEET AND PRODUCT THERE- 0 Thomas J. Fitzpatrick, Libertyville, IlL, assignor to United States Gypsum Company, Chicago, 11]., a corporation of Delaware Filed Dec. 24, 1964, Ser. No. 420,907 9 Claims. (Cl. 29-610) ABSTRACT OF THE DISCLOSURE Electrically-conductive sheets of improved resistive stability, from which radiant heating elements of predictable wattage output may be manufactured, are prepared by subjecting a gel-free aqueous mixture of alkali-stabilized colloidal silica sol, finely-divided graphite particles, wetting agent and pH modifier to extended mechanical working so as to individually and discretely coat the graphite particles with silica sol and form a homogenized dispersion, which is uniformly applied to a dimensionallystable supporting web, followed by drying at elevated temperature and heat stabilization; the resulting sheets being aged and formed into a radiant heating element by applying opposed electrodes.

This invention relates to electrically-conductive sheets of improved resistive stability from which stable radiant heating elements of predictable wattage output may be prepared. In a specific embodiment it also relates to an improved process for preparing electrically-conductive sheets and radiant-heating elements from conductive compositions comprising finely-divided graphite particles in a binder of colloidal silica sol.

Much effort has been expended in recent years towards the development of electrically-conductive coatings and sheets and electrical elements produced therefrom which may be connected to a source of electrical energy to form radiant-heating surfaces. These surface may be used, for example, in pliant form as pipe-wrap heaters or the like. They may also be supported or otherwise laminated to rigid supporting surfaces such as glass or heat insensitive plastic to form radiant heaters such as warming trays or space heaters. They may also be laminated or otherwise secured to gypsum wallboard, plywood or other room-forming partition material to form walls or ceilings with integrallydncorporated radiant-heating surfaces for human comfort heating.

One particularly-promising approach in this development are compositions comprising electrically-conductive particles such as graphite dispersed in a colloidal silica sol, as reflected in U.S. Patents 2,803,566 and 2,952,761. However, there have been particularly-vexing and heretofore-unsolved problems associated with the graphite-silica sol compositions. These include unpredictability of resistance from one coating batch to the next, variation in the resistance of the coating from one area to the next irrespective of uniformity of thickness, change of resistance when forming an electrical element from a coated surface, instability of the element resistance whatever it initially may be-with or without change of environmental temperature and humidity levelsand the low yield of usable element from attempted production runs. It is to the solution of these heretofore unsolved problems that the present invention is primarily addressed, as is apparent from the following objects.

tes Tatent G Patented Sept. 19, 1967 Objects It is therefore an object of the present invention to cope with the aforementioned problems and to provide a commercially-acceptable, radiant-heating element from graphite-colloidal silica sol compositions. It is a more specific object of the present invention to provide an electrically-conductive sheet and radiant-heating elements, the resistances of which are substantially predictable from the compositions thereof. It is another specific object to provide radiant-heating sheets which are stabilized as to resistance valueand thus wattage output at constant voltage-and are substantially unaffected by ambient conditions of temperature and humidity or frequency or length of usage. It is a further object of the present invention to provide a method for producing a low-cost electrically-conductive coating from graphite and colloidal silica sol, which coating is stable and uniform as to resistance and produceable in high yield on a high-volume production basis. These and other objects of the present invention will become apparent from the following detailed description, including the accompanying drawing, wherein:

The figure presents a correlation for determining the relative proportions of graphite and colloidal silica sol required to achieve a coating of a desired resistance in accordance with the present invention, as more fully discussed hereinafter.

T he invention The objects of this invention are achieved by an electrically-conductive sheet which is incapable of definition except by the process of making it. This process in one embodiment comprises providing a gel-free aqueous mixture comprising an alkalized-stabilized colloidal silica sol, electrolytic-grade graphite particles, a wetting agent for the graphite particles and sufficient pH modifier to prevent gelation of the resulting mixture during the subsequent steps of the process. These steps include subjecting the mixture to mechanical agitation or homogenization so as to break up any agglomerates formed therein without substantial pulverization of the graphite particles, substantially completely disperse the graphite particles uniformly throughout said colloidal silica sol and individually and discretely coat the graphite particles with the colloidal silica sol. The resulting highly-dispersed mixture is uniformly applied to a nonconductive, dimensionallystable supporting Web such as asbestos-containing paper so as to form a coating of substantially-uniform thickness. The coating is then dried at elevated temperatures and substantially immediately heat stabilized by raising the temperature thereof to about 250 F. to 375 F. for a period of at least about 5 seconds, e.g. about 5 seconds to 5 minutes. Details of each of these process steps are presented hereinafter.

Gel-free aqueous mixture and preparation thereof The type of alkali-stabilized colloidal silica sol preferably employed in the gel-free aqueous mixture of this invention is marketed under the trade name Ludox by E. I. du Pont de Nemours & Company and under the trade name Syton by Monsanto Chemical Company. Ludox colloidal silica is marketed as High-Sodium Ludox colloidal silica (H.S. Ludox) or Low-Sodium Ludox colloidal silica (L.S. Ludox), and either may be employed in the present process, although H.S. Ludox is preferred.

3 HS. Ludox colloidal silica and the method of making it are described in US. Patent 2,244,325, 2,574,902 and 2,597,872. LS. Ludox colloidal silica is described in U.S. Patent 2,577,485.

The preferred H.S. Ludox is a stable aqueous silica sol containing about 20 to 35 percent by Weight of SiO and having a silica-alkali ratio calculated as Na O of between about 60:1 to 130:1. It contains discrete silica particles which have a molecular weight, as determined by light scattering, of more than one-half million. The relative viscosity, at percent SiO varies from about 1.15 to 1.55 at 25 C.

As marketed, H.S. Ludox colloidal silica contains about 29 to 31 percent SiO about 0.29 to 0.39 percent Na O and a maximum of 0.15 sulfate as Na SO It is obtainable in the form of a water slurry containing about 30 percent solids. The silica particles are extremely small, ranging from about 0.01 to 0.03 micron in maximum dimension. The colloidal dispersion has an insolubilizing action on water soluble substances, e.g., water-soluble synthetic resins. As used in the coating composition of the present invention, the Ludox colloidal silica sol is diluted with water to achieve a desired final viscosity, as discussed hereinafter.

Any standard electrolytic-grade graphite, including colloidal and semi-colloidal graphite, graphite flakes, pulverulent graphite, or the like, may be employed in combination with the colloidal silica. The size of the graphite particles is not critical but they should have a maximum dimension substantially less than the desired thickness of the resultant dried conductive coating, which might typically vary from 1.5 to 2.0 mils in thickness but is not limited thereto. In a preferred embodiment the conductive particles are typically 1 to 10 microns in size and are prepared by the Acheson process. As a specific example, Acheson Grade 38 synthetic electrolytic-grade graphite powder, manufactured by National Carbon Company, has been employed with considerable success.

The relative proportion of graphite and colloidal silica sol in a coating composition is the primary variable which determines the electrical resistance of the resulting coating, assuming a constant thickness, the greater the proportion of graphite, the lower the resistance, and vice versa. Other variables such as the nature and absorption characteristics of the supporting web also must be considered and a method for determining the proportions of graphite and colloidal silica sol which allows for the effects of such other variables is presented hereinafter, in conjunction with the explanation of the figure.

The anionic wetting agent for the graphite particles is necessary to secure good dispersion of the particles in the colloidal silica sol during the subsequent extended mixing or homogenizing operation. Additional wetting agent or other surface-active agent may also be added after the extended mixing operation to defoam the mixture and maintain the uniformity of the graphite dispersion. The optimum amount of anionic wetting agent to achieve graphite dispersion and, if necessary, defoaming in any particular case depends upon several variables, including the particular wetting agent employed, the ratio of graphite to silica so] and the hardness of the water in the mixture, i.e., the concentration of calcium and magnesium ions in the water due to the presence of various salts thereof, e.g., carbonates, chlorides, sulfates and/or the like. In any particular mixture the amount required is best determined by experience, specific illustrations for particular mixtures being presented in the examples hereinafter.

Anionic wetting agents are preferred because they tend to neutralize the charges on the graphite particles and thus enhance dispersion. While non-i0nic wetting agents could be used, cationic types have been found to be generally unsuitable because they may cause precipitation of the colloidal silica sol. As a specific example of a pre ferred anionic Wetting agent, Aerosol OT-100 (American Cyanamid Company), comprising dioctyl sodium sulfosuccinate, a water-insoluble solid, may be dissolved in an equal weight of methyl Cellosolve acetate (abbreviated hereinafter MCA), the resultant solution being compatible with water. The amount of OT-lOO and MCA added to the aqueous coating solution is typically in the ran e of about 0.002 to 0.2 percent by weight. Where relatively soft water is employed, e.g., substantially less than 5 grains per gallon (a grain being equivalent to about 17.1 parts per million), considerable foaming may be encountered and as much as half or more of the total wetting agent may be added for purposes of defoaming. In the case of hard water, e.g., about 20-30 grains per gallon, very little foaming is encountered and relatively little, if any, Wetting agent is required for defoaming.

As already indicated, it is essential that the aqueous mixture of colloidal silica sol and graphite be substantially gel-free. The primary control in this regard is pH, although the method of mixing or dispersing the wetted graphite in the silica is also important, as hereinafter discussed. In general pH of the mixture is adjusted and maintained so that no gelation occurs throughout the subsequent processing steps including drying operations. This has been effectively accomplished by addition of a pH modifier in suflicient quantity to maintain a pH in the range of about 8.1 to 9.4 (for a 1 percent solution in water), preferably about 8.4 to 9.0. To assure maintenance of pH once the initial adjustment is made, the pH modifier should preferably be a permanent or non-fugitive type, that is, a modifier which will not evaporate with the application of heat as herein contemplated, that is, temperatures up to about 400 F. Accordingly, when using a non-fugitive pH modifier it is only necessary to make an initial pH adjustment, the maintenance thereof being assured.

Any conventional permanent-type alkali metal or alkali earth metal hydroxide may be employed as a pH modifier, e.g., calcium hydroxide, sodium hydroxide, potassium hydroxide, sodium silicate, or the like. In a preferred embodiment, commercial-grade sodium silicate is used, e.g., Dupont adhesive 90364 in aqueous solution, and for such embodiment it has been found essential to keep the weight ratio of sodium silicate to colloidal silica sol e.g., H.S. Ludox, within the limits of about 0.08:0.02. Because of their fugitive nature at elevated temperatures, pH modifiers such as ammonium hydroxide, organic amines, and the like, have been found to be unsuitable for purposes of the present invention.

As a complement to pH control, gelation is also avoided and undesired agglomerate formation is minimized by controlled addition rates during admixing of the ingredients, i.e., the colloidal silica sol, graphite, wetting agent and pH modifier. While the order of addition does not appear to be critical, it is essential that they be added to one another slowly and uniformly. By slowly and uniformly is meant that the ingredients are added continuously or incrementally over a substantial period of time, either simultaneously or separately, preferably separately. For example, the required amount of graphite might be added to the silica sol over a period of at least about 5 minutes, preferably 10 to 20 minutes, either substantially continuously and uniformly or in fractional increments, say one-fifth the total in 5 increments, over the same period of time, all while the silica sol is being agitated by stirring, mixing or the like. Similarly, the anionic wetting agent should be added slowly and uniformly over a period of not less than about 0.5 minute, e.g., about 0.5 to 5 minutes, typically about 1 minute. Likewise the pH modifier should be added slowly and uniformly over a period of at least 2 minutes, preferably at least about 4 minutes, e.g., 5 to 10 minutes, while the materials are being agitated.

In a preferred embodiment, the alkali-stabilized colloidal silica sol is placed in a vessel equipped with a mixer. While the silica sol is being agitated, the required amount of graphite powder is added in approximate onefifth increments, each increment being added only after the previous increment has become soaked. In practice, graphite addition is completed over a period of about to 20 minutes. After complete soaking of the graphite, the preferred anionic wetting agent is then slowly and uniformly added over a period of about 0.5 to 5 minutes, typically 1 minute, and mixing continued for at least 1 minute after the addition is completed, preferably about 2-3 minutes. The pH modifier, e.g., aqueous sodium silicate, is then added over a period of at least 2 minutes, preferably 5 to 10 minutes, while mixing is continued. The rate of addition of the aqueou sodium silicate can be speeded up during the addition period as the mix becomes more fluid. After formation of the admixture, the extended mixing operation follows, as hereinafter described.

As previously indicated, water is normally present in the mixture so that after the extended mixing operation the viscosity of the composition is satisfactory for uniformly applying the coating to a supporting web on a highspeed continuou basis. In practice, a Zahn Cup viscosity at the time of coating in the range of about 7 to 10 seconds, using a No. 4 Zahn Cup at ambient conditions, preferably 8.0 to 9.5 seconds, has been found satisfactory. The Zahn Cup viscosity-measuring technique is described, for example, in Gardner-Sword 1962 Paint Testing Manual, 12th edition, at page 184. It may be converted, if desired, to other standard viscosity units, e.g., centipoises, by use of available correlations. If the viscosity measurement is made immediately upon termination of the extended mixing or homogenizing operation, it should be recognized that the mixing operation may raise the temperature of the mix as much as 20 or more above ambient conditions, e.g., from 75 F. to 95 F. If the mix is allowed to cool substantially before the coating operation, such factor should be allowed for in formulating the water content of the mix.

The water is conveniently added initially to the colloidal silica sol before addition of the graphite, although small increments of water may also be added to the final mix for minor viscosity adjustments. A portion of the water is also advantageously added to the pH modifier, e.g., sodium silicate, to assist in blending the pH modifier uniformly into the mix. In practice, for example, an amount of water equal to the weight of pH modifier may be added to the pH modifier before addition thereof.

While premixing of water with the pH modifier has proved advantageous in achieving a well-dispersed mix without excessive agglomeration of ingredients, such technique surprisingly is not operable when adding the graphite particles to the colloidal silica sol and will result in an unsatisfactory product. It is postulated, but without limitation, that when a limited amount of the colloidal silica sol is premixed with the graphite each particle of graphite is coated with a layer of hydrated polysilicic acid. When this is dispersed in excess colloidal silica sol and subsequently subjected to elevated drying and curing temperatures as hereinafter described, the coating is broken away leaving uncoated graphite particlesin contrast to the desired coated particles. Whatever the explanation, the direct and controlled addition of the graphite to all or the bulk of the colloidal silica sol results in the requisite coated particles found essential to stability; and the premixing technique does not.

Homogenization of the mixture Once the mixture, i.e., aqueous colloidal silica sol, graphite, aqueous sodium silicate and MCA/OT-lOO, is prepared, it must be subjected to extended mechanical agitation of a homogenization character, usually for a period in excess of 15 minutes, preferably about minutes to 1 hour, optimally about -35 minutes. This agitation is designed to break up agglomerates formed by the ingredients in the mixture without pulverization of the 6 individual graphite particles. In addition, it must substantially completely disperse the graphite particles uniformly throughout the silica sol and at the same time individually and discretely coat them with silica sol.

This homogenization type of agitation may be obtained by use of a high-speed, high-shear, turbine-stator mechanism of the type capable of creating stable-emulsions and suspensions. The turbine-stator mechanism is typically disposed vertically in the mixing vessel and immersed in the mixture. The clearance between the turbine and stator is preferably fixed, and the turbine rotates at a speed that develops a pressure differential between the bottom of the turbine and the surface of the material being homogenized. As a result, liquid is continuously drawn from the bottom of the container and forced to pass through the restricted openings of the mixer head where it is subjected to shear forces which break agglomerates down to primary particle size without pulverization and intimately and repeatedly contact the individual particle surfaces with the silica sol. Since the turbine also acts as a pump, the material is discharged in an upward direction after it has passed through the homogenizing zone. An adjustable baffle above the zone then deflects this rising current toward the container sides which, in turn, direct the material back into the mixing area where the cycle is again repeated.

A particularly advantageous homogenizer for use in the practice of the present invention is the Eppenbach Homo- Mixer manufactured by the Eppenbach Division of Gifford-Wood Co., 420 Lexington Avenue, New York 17, New York (factory at Hudson, New York). Both the Eppenbach Model 1-HU and Model 4H Homo-Mixers have been employed successfully, and specifications for these models are presented in Gifford-Wood Co. Catalogue No. 600, Copyright 1957. For the Model l-HU they include a shaft speed of 7200 rpm. and a peripheral speed of 61 feet per second; for the Model 4H they include a shaft speed of 3500 r.p.m. and a peripheral speed of 67 feet per second. Other models of Eppenbach Homo-Mixers are also suitable, the basic difference in models being primarily capacity rating. When using Eppenbach Homo- Mixers, it has been found that the best homogenization of the mixture has occurred when a ratio of the mixer head diameter to the container diameter is in the neighborhood of about 10 to 30 percent, preferably about 15 to 20 percent.

Determination of adequate homogenization or dispersion can readily be made by dropping a small sample of the mixture, e.g., 1 cc., into a beaker of distilled water, e.g., 400 cc., and observing the settling characteristics. When there has been inadequate dispersion, the particles appear to coagulate, precipitate or otherwise fioc and rapidly settle to the bottom, more than half the particles typically settling in less than 15 minutes. In contrast, in case of a properly dispersed mixture, no substantial settlement may occur within an hour and less than half may settle in 24 hours. In practice, the mixing of the material is preferably carried out for about 3035 minutes at which time a settlement-characteristic check is made. If rapid settling out of the particles is detected, the mixing operation is resumed until a subsequent check indicates complete dispersion.

Once complete dispersion of the graphite particles has been obtained, it appears that additional mixing has no substantial further benefit. For example, no difference could be found between the same compositions completely dispersed in 30 minutes and wherein homogenization was deliberately continued for a total of minutes.

As an indication of the criticality of breaking up agglomerates, completely dispersing the graphite particles without pulverizing them, and individually and discretely coating them with colloidal silica sol, conventional mixers lacking homogenizing capabilities were tested and found to be wanting. For example, a Lightnin Mixer Model 75238, Type 12A, a high speed, lower shear mixer having four blades of approximately 4 inches in length revolving at 1725 r.p.m., did not obtain complete dispersion of the graphite particles in the colloidal silica sol even after an extended mixing period of two hours. A T urbo-Mixer 4-10, a relatively high-speed mixer revolving at 2000 rpm. but having relatively large clearances between impeller and stator, exhibited insufiicient shearing action and failed to adequately disperse the graphite. A Morehouse Colloidal Mill Model SVTVX (5 HP, variable drive) of the spinning-stone type, in which one stone remains stationary and a second stone spins outside the first stone with tolerances down to a few thousandths of an inch, pulverized rather than dispersed the graphite particles. Still another mixer used and found unsatisfactory was a Cowles Dissolver Model MI, which swirled the graphite particles away from the mixer blade resulting in poor dispersion.

Upon completion of homogenization, a surface active agent, e.g., the same MCA/OT-100 solution, may, as aforementioned, be added to defoam the mixture, particularly when soft water is employed in the mixture. The amount will depend upon the degree of defoaming required by may approximate .002 to 0.2 percent by weight, e.g., 0.015 percent. The final addition of the wetting/ defoaming agent may be distributed in the mixture by running the homogenizing mixer an additional 0.5 to 3 minutes or until defoaming is complete, preferably with a low bafiie setting to minimize violent agitation. As a precaution the defoamed mixture may then be filtered through a fine mesh screen, e.g., a 20 mesh screen (U.S. Sieve Series), into another clean container in preparation for application to a supporting web or substratum. This final filtering is designed to remove any residual agglomerates and/ or dry out deposits which may flake ofif the side of the homogenizing vessel.

While the defoarned mixture exhibits a high degree of dispersion for a substantial period of time, running into days, undefined and undesired changes therein dictate its use Within about 12 hours, preferably within about 6 hours. Optimally, the coating composition is used substantially immediately after preparation, although such timing is not critical. If the coating is not used promptly, it should be kept at room temperature, e.g., about 70 F., preferably with mild agitation, which may continue during the subsequent coating operation. During this lowenergy agitation additional Wetting agent, e.g., 0.005 percent, may be added to minimize air entrapment.

Supporting web The type of supporting web is not critical so long as it is noncond-uctive, not incompatible with the coating solution, not adversely affected by the elevated temperatures to be employed during subsequent processing or use and yet meets other hysical demands to be made upon it, e.g., strengthwise, pliabilitywise and the like. In addition, the support web and conductive coating must bond to one another, and must not differ substantially in dimensional response to changes in moisture content, temperature and the like.

While the conductive coating of the present invention may be applied directly to rigid support webs, it is described hereinafter in connection with pliant support Webs suitable for subsequent lamination to room-forming partition materials, e.g., gypsum wallboard. It should be understood, however, that the invention is not limited to such application and that those skilled in the art, in the light of the present disclosure, are capable of readily adapting it to other desired applications.

Since the conductive coating of the present invention has a substantially negligible coefiicient of dimensional change with respect to moisture content, temperature, and the like, the pliant supporting web should be similarly dimensionally stable. In practice, the severest dimensional change has been found to occur when drying and curing the web after ap lication of the liquid coating thereto.

Accordingly, it has been found necessary to select a supporting Web material which will not change more than 3 percent in any dimension from a totally soaked condition to a substantially bone dry condition, preferably one which will change less than 1 percent, optimally less than 0.2 percent.

Suitable supporting materials include certain dimensionally-stable cellulosic papers (but not ordinary kraft paper or the like), mineral-fiber sheets such as asbestos sheets, synthetic-fiber sheets such as fibrous glass, Dacron and nylon, and the like, including combinations thereof. Because of its excellent dimensional stability, low cost, ready availability and fire-resistant properties, asbestoscontaining sheets are preferred, reinforced, if necessary, to achieve an adequate machine-direction wet strength, e.g., at least about 15 pounds per inch of Width. In a preferred embodiment, a two-ply, 7-mil-thick sheet known as coating stock Grade 4029, manufactured by the Blandy Paper Division of Hollingsworth and Vose, Schuylerville, New York, is advantageously employed. One ply comprises asbestos fibers and is the ply to which the coating is applied. The other ply comprises glass and cellulosic fibers (along with a minor proportion of asbestos fibers for interply strength) and provides the necessary wet tensile strength to the stock. It also, incidentally, provides a smooth and attractive outer surface which can be imprinted With trademark indicia or the like, and for this reason this strengthening ply is preferably faced outwardly in the event of subsequent lamination.

Bonding between the conductive coating is in part achieved by entry of the coating composition into the pores of the asbestos supporting web by capillary action or the like. Surprisingly, this is a selective process involving primarily the silica sol. If allowed to continue for a substantial period of time, it can result in the removal of substantial amounts of silica sol (as much as 25-50 percent or more) from the coating composition, particularly when employing paper of high absorptivity, thereby substantially changing the ratio of graphite to silica sol and hence the resistance of the resulting coating. Moreover, if the absorptivity of the supporting web varies from area to area, a not-uncommon characteristic of commercially-produced stocks, the rate of absorption of silica sol will vary, resulting in undesired variations in the composition of the conductive coating from area to area.

These problems, now fully recognized, are inexpensively coped with by treating or priming the commercial grade stock to achieve a uniform and lower absorptivity and subjecting the primed stock, as soon as coated with the conductive coating, to drying conditions substantially immediately, e.g., within about 20 seconds, preferably less. Thus, drying is substantially complete within about 1 minute or so and no substantial amount of colloidal silica sol is separated from the graphite. In the case of the aforementioned Blandy Grade 4029 supporting web, the asbestos-ply side is primed with a rubber-type priming agent such as Bondmaster 379-20 (Bondmaster Division of the Rubber and Asbestos Corporation). The priming operation simply involves passing the asbestos-ply side of the paper over transfer rolls, e.g., 60-pad transfer rolls, to apply a 34- mil wet coating and drying at moderate temperatures, e.g., about 200 F.-250 F. The primed web typically will have a thickness of about 8 mils, i.e., 1 mil thicker than the unprinted web.

After priming, the web should have a substantiallyuniform absorptivity, e.g., about 410 minutes, preferably about 68 minutes, as measured by a Water-drop absorption test. This simple test comprises placing a drop of water on the primed web and timing how long it takes for absorption into the web. The water used in the test is prepared by adding 6 drops of MCA/ OT- Wetting agent, already described herein, to a liter of distilled water. In addition to cont-rolling absorptivity, priming of the supporting web also minimizes attack on the asbestos by the highly-alkaline conductive coating.

Priming to obtain an absorptivity of about 6-10 minutes implicitly assumes that the asbestos web has an absorptivity before priming substantially greater (lower drop-test minutes). In practice, the desired final absorptivity before priming substantially greater (lower to 8 minutes, is normally obtained after priming if the absorptivity of the unprimed stock is no lower than that corresponding to about 2 drop-test minutes.

In the case of supporting webs of uniform absorptivity, priming may be avoided, particularly if the materials comprising the web are not attacked by the coating composition and if drying of the web follows substantially immediately after coating. In the case of unprimed webs an absorptivity of 4 to 10 minutes is also desired, although higher and lower absorptivities can be employed if coating conditions are carefully controlled. In the case of higher absorptivities it may become necessary to adjust the initial graphite-colloidal silica sol ratios to take into consideration the amount of silica sol absorbed into the web.

The correlation of resistance and graphite-silica sol ratio presented in the figure, to be discussed hereinafter, assumes that the web is primed asbestos and has an absorptivity of about 6 to 8 minutes, and that the coating is dried substantially immediately after being applied to the web. The correlation can, however, be readily adjusted for other supporting webs (substrata) and conditions, as will become apparent.

Uniformity of coating In addition to homogeneity of the coating composition and uniformity of absorptivity of the supporting web, it is also essential that the coating be of uniform thickness. In addition, the thickness of the coating should not be so great as to be affected by the normal flexure of the Web when wound on to conventional-size rolls such as may be employed in the subsequent handling thereof, e.g., a 6-inch O.D. roll. In practice, a coating having a uniform wet thickness in the range of about 5 to 8 mils, preferably about 6 to 7 mils, and a dry thickness of about 1.5 to 2.0 mils has satisfactorily met the requirements. The correlation of the figure has been prepared for the preferred Wet thicknesses of 6 and 7 mils, but it can be employed for other thicknesses by adjustments as hereinafter described.

Various conventional techniques may be employed to apply a coating of uniform thickness, as those skilled in the art of coating will recognize. Such technique include, for example, the use of roller coaters, e.g., Egan-type roller coater, trailing-blade coating machines and airknife coating machines.

Viscosity of the coating is a critical variable in achieving a satisfactory coating operation by such means. As described hereinabove, viscosity should be in the range of about 7 to 10 seconds for a No. 4 Zahn Cup (or 9 to 12 seconds for a No. 3 Zahn Cup). Water content of the coating composition is the primary viscosity control, most if not all of the water being added with the colloidal silica sol and the pH modifier. Any minor viscosity adjustments in the direction of decreasing viscosity may be made after formulating and homogenizing the composition by admixing carefully controlled increments of water.

Drying and heat stabilization As previously indicated, drying of the coating is effected substantially immediately after application of the coating composition to the asbestos supporting web to prevent excessive absorption of the colloidal silica sol. This is effected by subjecting the coated web to elevated temperatures, e.g., 250 F. to 450 F., whereby drying is substantially complete within about 1 minute.

To achieve stability of resistance, the dried coating must also be heat cured substantially immediately after drying, i.e., within about 5 minutes, preferably Within 1 minute, by raising its temperature still further to 250 F. to 375 F., preferably 275 F.. to 325 F., for about 5 seconds to 5 minutes, preferably 10 seconds to 1 minute, the longer periods being employed at the lower temperatures. Curing periods substantially less than 5 seconds are generally ineffective, whereas curing times substantially in excess of about 3-5 minutes may overcook the coating, resulting in excessive dehydration of the colloidal silica sol and complete electrical instability of the coating. If the curing step is delayed excessively after the drying step, presently-inexplicable changes apparently take place, adversely affecting electrical stability of the final coating.

In a preferred embodiment, the drying and heat stabilization steps are carried out as a continuous operation by passing the coated web continuously through an oven with zone temperatures regulated therein whereby the coating on the exiting web has reached a temperature in the range of 275 F. to 325 F., e.g., 300 F. In a particular embodiment for example, the successive treating steps are carried out in a -foot oven at 425 F. through which an 8-mil primed asbestos web with a 6-mil wet coating, as previously described, passes at a rate of about 60 feet per minute. The dried and cured coating at exit has a temperature of about 300 F. For coatings of different thicknesses and/or compositions, adjustment in drying/ curing conditions may be required as experience dictates.

Formation of electrical element An electrical element is formed from the coated web 'by securing opposed electrodes or bus bars to the surface of the coating, which electrodes in a specific embodiment comprise, for example /2-inch wide copper strips having a thickness of .003 inch and extending the length of the coated web adjacent opposed edges thereof. Advantageously, the opposed electrodes are adhesively secured and at the same time a protective asbestos overlay sheet is adhesively applied to the coated web by passing the coated web and overlay sheet with electrodes therebetween through nip rolls, a bank of adhesive being present ahead of thenip rolls. The asbestos overlay sheet may comprise a 7-mil thick conventional asbestos sheet such as, for example, Nicel (Nicolet Industries, Florham Park, New Jersey). The adhesive may comprise, for example, a blend of about 25 percent H.S. Ludox and about 75 percent Dylex K55 (a styrene-butadiene rubber emulsion marketed by the Koppers Company).

While at several stages of the manufacture of the coated web, it has been found essential to proceed with the next step substantially immediately or without substantial aging, quite surprisingly and inexplicably, the formation of the electrical element therefrom, as above described, should preferably be substantially delayed, e.g., at least about 6 hours, preferably at least about 12 hours, e.g., 24 hours or more, to achieve optimum results. Failure to age the green coated sheet before combining with electrodes and overlay sheet can result in electrical instability and/ or resistances outside predicted values.

Use of the figure As previously indicated, the primary control for arriving at a given resistance in the resulting electrically-conductive coating is the relative proportions of colloidal silica sol and graphite in the coating composition. As the result of tests of numerous coating compositions, correlations relating the proportions of graphite and colloidal silica sol with resistance of the resulting coating have been developed. These are presented in the figure and represent linear regression curves arrived at by leastsquares analyses of available data. They show on the ordinate the resistance per square of coatings as related to the amount of graphite in grams (abscissa) in a standard liquid containing 2400 grams of HS. Ludox colloidal silica sol. The curves are for wet coating thicknesses of 6 and 7 mils respectively, which thicknesses have been found to be particularly advantageous.

1 l The standard liquid is assumed to have the following composition, which, after graphite addition, satisfies the various requirements for coating compositions hereina'bove described, and is prepared as hereinabove set forth:

Grams H.S. Ludox 2400 Water in the HS. Ludox Sodium silicate 200 Water in sodium silicate 200 MCA 1.0 OT-O in MCA 1.0

Adjusted to yield viscosity of 8 seconds using No. 4 Zahn Cup for homogenized coating composition.

It is also assumed that the coating composition is applied to primed Blandy Grade 4029 coating stock or equivalent of 6-8 minute water-drop" absorption and is dried and cured in accordance with procedures hereinabove described.

If minor variations are introduced into the coating composition, the type of supporting web or the precise procedures set forth herein, the proper ratio of graphite to colloidal silica sol for same may be readily arrived at by preparing a check sample using the proportions indicated by the regression curve and measuring the actual resistance thereof in ohms per square. If it does not correspond to the desired resistance, the actual resistance should be plotted" on the correlation, a line drawn therethrough parallel to the existing lines, and the proper amount of graphite per 2400 grams of colloidal silica sol read at the intersection of the new line with the desired resistance ordinate.

For example, if it were desired to prepare a heating element having a resistance of 70 ohms per square using a supporting web of unknown absorptivity or absorptivity differing from that of the web described herein, and employing a wet coating thickness of 7 mils, the regression curve of the figure would indicate that about 1100 grams of graphite should be added for 2400 grams of colloidal silica sol. If after preparing the element as above described, the actual resistance was found to be 100 ohms per square, instead of 70, this would be plotted on the figure, as indicated at A, a parallel line would be drawn therethrough and the correct proportion of graphite read where the line intersects the 70-ohm ordinate, as indicated at B, i.e., about 1170 grams of graphite per 2400 grams of colloidal silica sol.

While amounts of graphite in the figure are expressed in grams per 2400 grams of colloidal silica sol (H.S. Ludox), these amounts are intended to indicate proportions. It should be understood that the amounts may be scaled up or down as desired. The resistances on the ordinate of the figure are expressed in ohms per square, rather than, for example, ohms per square inch, or per square foot, or per square yard or the like. For constant-thickness coatings the resistance per square inch of surface, as measured from opposed edges, is the same as the resistance per square foot, per square yard, etc., and thus units need not be expressed.

Use of electrical element with gypsum wallboard While the electrical element above described has a variety of uses as already indicated, e.g., pipe wrap, warming trays, space heaters and the like, one of its largest potential markets is in the area of human comfort heating as a radiant heating surface integrally-incorporated with gypsum wallboard or similar support structure, although the invention is not limited thereto. In a specific embodiment, the heating element of appropriate resistance, including supporting web with conductive coating, copper electrodes and asbestos overlay sheet as above described, is adhered to one surface of gypsum wallboard, e.g., conventional /2-inch thick gypsum wallboard having a density of 1700 to 2100 pounds per thousand square feet, by means of, for example, a commercial-grade sodium silicate adhesive, preferably containing a minor proportion of wetting agent. A preferred adhesive is Dupont mineral adhesive 903-64, although other compatible adhesives may also be used.

The resulting wallboard with integrally-incorporated radiant heating element may then be installed as room partitions and ceilings, although in a preferred embodiment only the ceiling need contain the radiant heating element. The wallboard with the heating surface on the room side of the board may be glued, nailed, or otherwise secured to framing, joists, or the like, although when employing metallic securing devices such as nails, insulating collars, e.g., nylon, are preferably employed for added safety. Since the heating element is in effect a large area parallel resistor, holes or other apertures may be cut into the board, as with conventional wallboard, for utilities, e.g., electrical wiring, ceiling fixtures and the like. After securing the wallboard to the supporting structure, electrode connectors are soldered, clipped or otherwise fastened thereto so as to make good, permanent electrical contact with the electrodes (which may be exposed if desired or necessary by scraping and/or peeling back the asbestos covering or the like). The electrode connectors are then hooked into the building power supply, e.g., conventional 120 volt A.C. house current, with appropriate thermostatic controls, usually separate controls for each zone.

After installation, the wallboard may receive conventional finishing treatment, e.g., taping and painting, but in a preferred embodiment a thin overlay board may be cemented thereto on the room side before finishing treatment, primarily to provide electrical-insulative protection for the conductive laminate. In such embodiment the overlay board should obviously have good heat-transfer characteristics and thus may comprise, for example, fii-inch gypsum wallboard with high-density gypsum core (e.g., 1250-1400 pounds per thousand square feet). The cement may comprise a contact adhesive applied to both surfaces, for example, a rubber-based cement dissolved in methyl ethyl ketone, such as Acorn Drywall Cement (Acorn-Wilhold Adhesive Company, Chicago, Illinois).

In another embodiment, the electrical element may be substituted for one of the paper covers at the time of manufacture of the gypsum wallboard, rather than being subsequently laminated thereto. In other embodiments the electrical element may be sandwiched between two gypsum wallboard cores, preferably one conventional low-density and the other high-density, to form a prefabricated laminate with the electrical element protected on each side. The outer wallboard layers may have the same thickness, e.g., fii-inch thick, or may differ in thickness, in which case the thinner layer should face the space to be heated and should preferably have a high density for improved heat transfer. Conventional insulation, preferably 6 inches or more, is normally employed to minimize heat loss in the direction away from the space to be heated.

The present invention will be more clearly understood from the following specific examples.

EXAMPLE 1 To prepare an elongated electrical element having a width of 46 inches with /z-inch wide, .003-inch thick electrodes adhered 41-inch from the edges thereof, leaving an effective conductive path of 44% inches between the inner edges of the electrodes, it has been determined that in the case of a ceiling installation the uniform heat output should be about 59 Btu. per hour per square foot or about 17.3 watts per square foot. Such heat output is sufficient to properly heat a space for human comfort, assuming an inside-outside temperature differential of F., e.g., inside temperature of 70 F. and outside temperature of 10 F. With such heat output, surface temperatures of the radiant heating panel will be less than 120 F., e.g., about F. F., well below temperatures which might cause gypsum calcina 13 tion problems, paper charring, paint deterioration, human discomfort upon momentary contact, or the like.

The electrical resistance to approximate this heat output is defined by the formula:

R E X 144 where R is the resistance in ohms per square, E is the voltage in volts (considered in this example to be 120 volts to correspond to ordinary 120-volt A.C. house current), d is the distance between electrodes in inches and Q is the desired heat output in watts per square foot. In the present example the necessary resistance is about 60 ohms per square.

To prepare about a 40-gallon batch of conductive coating for such purposes in accordance with the present invention, employing the relationship of the figure and assuming a wet coating thickness of about 6 mils, the following formulation is arrived at:

The 234 pounds of HS. Ludox is placed in a clean SS-gallon drum and the 54 pounds of water, which in this specific example is soft water having a hardness content of only about 1 grain per gallon or less, added thereto. A Model 4-H Eppenbach Homo-Mixer is inserted with the battle set 2 inches to 3 inches below the surface of the liquid and started. The graphite is then added in increments of about 25 pounds (21 pounds for the last increment), the next increment not being added until the previous increment is thoroughly soaked. To draw graphite into the mixer, it may be pivoted slowly and smoothly around the drum. The baffle plate may also be raised and lowered very slowly to aid in soaking the graphite. Total elapsed time for the graphite soaking operation is typically about 12 to 18 minutes.

As soon as all the graphite is soaked, the first 30 grams of MCA/OT100 (equal weights of each) are slowly and uniformly added, e.g., over a period of 0.5 to 5 minutes, typically about 1 minute, while running the mixer. After MCA/OT-IOO addition and running the mixer an additional 2 to 3 minutes the admixture of pH modifier and water is slowly added, e.g., over a period of about 5 to minutes. The pH modifier renders the mixer more fiuid and as the fluidity increases the baffie plate should be lowabout 8.4 to 9.0 and remains so through subsequent dry- 7 ing.

The resulting gel-free aqueous mixture is then subjected to extended high-shear (mechanic-a1) working by running the mixer continuously for 30 minutes, after which the settlement-characteristic check, as hereinabove described, shows the mixture to be adequately dispersed. The mixture is then filtered through a ZO-mesh screen (U.S. Sieve Series) into a second clean drum, the temperature thereof being about 90 F.95 F. and the viscosity about 8 seconds using a No. 4 Zahn Cup.

The conductive coating is then applied by means of a roller coater to an 8-rnil thick, 46-inch wide substratum (supporting web), i.e., Blandy coating stock Grade 4029 primed with Bondmaster 379-20, the substratum having an absorptivity of about 6-8 minutes by the water-drop test hereinabove described. The coated substratum having 7 a 6-mil thick wet coating thereon is then immediately (less than 5 seconds) passed into an oven at 425 P. where during a residence time of about 1 minute, 40 seconds, it is dried and raised to a temperature above 250 F. for at least about 30 seconds, the temperature of the coating itself at exit being about 300 F. A check of resistance shows the resistance to be approximately 60 ohms per square.

The conductive web is then aged for about 24 hours, at which time it is converted to an electrical element by passing it, the electrodes above described and an overlay asbestos web, i.e., 7-mil-thick Nicel-l30 sheet, between nip rollers using a blend of 25 percent H.S. Ludox and percent Dylex K55 as the adhesive. A recheck of resistance again shows a resistance of approximately 60 ohms per square. The resulting electrical element is then centrally adhered by means of Dnpont mineral adhesive 903- 64 to the paper on one side of conventional /2-inch thick, 48-inch wide, 8-foot and 12-foot long gypsum building boards, the Nicel-l30 sheet facing inwardly and the electrodes running lengthwise.

The resulting building boards are nailed to ceiling joists of a room under construction by nails with nylon insulating collars. 011 each board one end of each electrode is exposed for about a distance of l-2 inches by scraping and peeling back. Electrical contact is then made thereto by wire clips, which in turn are connected to the 120-volt power supply with appropriate thermostatic controls. A Ai-inch high-density overlay board is cemented by means of Acorn Drywall Cement (Acorn Wilhold Adhesive Company, Chicago, Illinois), and 6 inches of rock wool insulation is added between the ceiling joists.

Upon energization of the board, the desired heat output of about 17.3 watts per square foot is obtained, indicating resistance remains at the desired level of 60 ohms per square. No significant change, e.g., more than 5 percent, in heat output is observed after prolonged usage, including repeated cycling between F./90% RH and 75 F./ 10% RH. By employing compositions and following procedures herein set forth, substantially duplicate conductive sheets and electrical elements can be produced in high volume and high yield with only minimal off-specification product.

If the applied voltage had been, for example, 230 volts, the required resistance, as computed by the formula set forth in the first paragraph of this example, would be about 220 ohms and the proportions of graphite to H8. Ludox would be changed as indicated in the figure. Water content would also be adjusted, if necessary, to achieve the desired viscosity of the homogenized mixture. Similarly, the sodium silicate pH modifier would be adjusted, if necessary, to prevent gelation by keeping the pH in the range of 8.1 to 9.4 while maintaining the weight ratio of sodium silicate to colloidal silica sol within about 0.08:0.02. The amount of wetting agent might also be changed as experience dictates to achieve desired wetting and def-oaming, although the amounts of wetting agent are usually not critical. Those skilled in the art will recognize, in the light of this disclosure, the adjustments necessary to achieve the desired result whatever the desired heat output, power supply, distance between electrodes, or the likeall within likely-to-be-incurred limits, of course.

EXAMPLE 2 A conductive coating and electrical element is prepared as described in Example 1 except that it has a width of 22 inches, in contrast to 46 inches, and an effective conductive path of 20% inches between the inner edges of the electrodes, in contrast to 44% inches. For the same heat output as in Example 1 when using a -volt power source, the required resistance, as calculated by the formula set forth in the first paragraph of Example 1, is about 277 ohms per square.

To prepare about a 35-gallon batch of conductive coating for such purposes, employing the relationship is of the figure and assuming a wet coating thickness of about 65 mils, the following formulation is arrived at:

Ingredient Weight Approx.

Wt. percent 206 lbs 25 grams ...:I

A coated asbestos web is prepared in the same manner as described in Example 1 and has a. resistance of about 270-280 ohms per square. An electrical element produced from the coated asbestos web, also as described in Example 1, has the same resistance, i.e., about 270 280 ohms. After lamination to a gypsum wallboard and installed as a ceiling radiant heating panel, as described in Example 1, the desired heat output is obtained with no change after prolonged usage and repeated cycling, even under extreme temperature and humidity variations. The product can be produced in high volume and high yield with minim-a1 oft-specification product.

EXAMPLE 3 A number of tests were made to illustrate the eflect of certain variables, e.g., the use of a highly-absorptive asbestos paper as the supporting Web, the effect of pH, the use of both permanent and non-permanent pH modifiers, and the effect of adding the pH modifier after the homogenization step. In these tests the compositions included 2400 parts by weight of HS. Ludox colloidal silica sol, 500 parts by weight of Acheson Grade 38 electrolytic graphite, 1 part of Aerosol OT-100 in 1 part by weight of a MCA diluent, 100 parts by Weight of water and the amounts of pH modifier (commercial grade sodium silicate unless otherwise indicated) shown in the following tabulation. The supporting web was highlyabsorptive asbestos paper, i.e., unprimed Blandy Grade 4029 coating stock having a water-drop absorptivity of about 1 minute.

The objective in each case was to secure a coated paper resistance of about 60 ohms per square within set limits of about 55 to 65 ohms per square, the wet coating thickness being about 6 mils. The test results are as follows:

1 Below.

2 200 parts of pH modifier added after homogenization. 3 23 parts NaOH to 152 parts H20.

4 124 parts NHiOH.

Note that in Tests 1 through 4, where the pH was maintained at about 8.5, the resistances of the coated asbestos papers were in the desired range of 55 to 65 ohms per square, averaging 61.9 ohms per square. It should also be particularly noted that these resistances were obtained With only 500 grams of graphite per 2400 grams of HS. Ludox, whereas the figure would indicate that about l235 grams of graphite are required. This illustrates the effect of high absorptivity on the graphite- Ludox ratio. A substantial portion of the Ludox, as much as half or more, is absorbed .by. the web, drastic-ally afiecting the ratio. In preferred embodiments already described, low absorptivity asbestos paper is used, e.g., primed Blandy Grade 4029 coating stock having a waterdrop absorptivity of 6 to 8 minutes, to avoid such efiect.

In Test 5, where the pH was adjusted to 9.5, the resistance approached 1000 ohms per square and was completely unacceptable. In Test 6, insufficient pH modifier was added, and the resultant coated paper resistance dropped to 504 ohms per square, substantially below specification.

In Test 7, where no pH modifier was used, the coated paper resistance dropped to an unacceptably low level of 22.7 ohms per square. In Test 7A, where pH modifier was added to a portion of the mix of Test 7 after homogenization thereof, the coated resistance averaged 96.6 ohms per square. This high resistance resulted even though the batch had the same chemical composition as the batches of Tests 1 through 4, clearly illustrating the critically of procedural sequence and evidencing the need for claiming the inventive product by the process of producing same.

The mix of Test 8 was prepared using a different permanent-type pH modifier, dilute sodium hydroxide, in sufiicient amounts to secure a pH of 8.5. As noted, a resistance of about 60 ohms per square was obtained, indicating that permanent pH modifiers other than sodium silicate can be used so long as the pH is maintained between about 8.1 and 9.4, preferably between about 8.4 and 9.0.

The mix of Test 9 demonstrates the efiect of using a non-permanent pH modifier. Ammonium hydroxide was chosen as the pH modifier knowing it to be fugitive at heat-curing temperatures of about 300 F. or higher, which temperatures were used to heat treat the coating. Sufficient ammonium hydroxide was initially added to obtain a pH of 8.5, yet the resistance dropped to 53.4 ohms per square from the average of 61.9 ohms per square obtained with the 8.5 pH of Tests 1 through 4. Accordingly, it is apparent that at the upper heat-curing temperatures a permanent pH modifier should be employed.

Conclusion From the above description, including specific examples, it is apparent that the objects of the present invention have been achieved. A uniform, stable electrically-conductive coating composition is provided, which, when applied in a film of uniform thickness upon a dimensionally-st-abie substratum, will yield a conductive heating element with a resistance which is uniform, predictable and non-shifting and with element characteristics such that the element can be readily laminated or adhered to another surface without significant shift of resistance. The coated substratum is relatively inexpensive to produce, lends itself to high-volume commercial production techniques and can be produced in high yield with minimum oft-specification product.

While the present invention has been described in connection with certain embodiments, it should be understood that the invention is not limited thereto. Alternative modifications of the present invention will be apparent from the above description to those skilled in the art and such modifications are considered as within the spirit and scope of the present invention and are intended to be covered in any patent based in any Way hereon.

Having thus described the invention, what is claimed 1s:

1. A resistively-stable, electrically-conductive sheet suitable for radiant-heating elements comprising a nonconductive, dimensionally-stable web supporting a conductive film, said film being prepared by the method which comprises:

(a) providing a gel-free aqueous mixture comprising an alkali-stabilized colloidal silica sol, electrolyticgrade graphite particles, a wetting agent for said graphite particles, and sufi'icient pH modifier to prevent gelation of the resulting mixture during the processing steps hereinafter set forth;

( b) subjecting said mixture to extended mechanical working including shear stresses so as to break up agglomerates formed by the ingredients of said aqueous mixture without substantial pu lverization of said graphite particles and to substantially-completely disperse said graphite particles uniformly throughout said colloidal silica sol and coat the graphite particles individually and discretely with said colloidal silica sol, whereby a substantially homogenized dispersion results;

(c) applying the resulting dispersion uniformly to said nonconductive, dimensionally-stable supporting web so as to form a coating of substantially-uniform thickness;

(d) drying the coating at elevated temperature; and

(e) substantially immediately heat stabilizing the coating by raising the temperature thereof to the range of about 250 F. to 375 F. for a period of about five seconds to five minutes.

2. A process for producing a resistively-stable, electrically-conductive sheet comprising the steps of:

(a) providing a gel-free aqueous mixture comprising an alkali-stabilized colloidal silica sol, electrolytic-grade graphite particles, a wetting agent for said graphite particles, and sufiicient pH modifier to prevent gelation of the resulting mixture during the processing steps hereinafter set forth;

( b) subjecting said mixture to extended mechanical agitation including shear stresses so as to break up agglomerates formed by the ingredients of said aqueous mixture without substantial pulverization of said graphite particles and to substantially-completely disperse said graphite particles uniformly throughout said colloidal silica sol and coat the graphite particles individually and discretely with said colloidal silica sol, whereby a substantially homogenized dispersion results;

, (c) applying the resulting dispersion uniformly to a nonconductive, dimensionally-stable supporting web so as to form a coating of substantially-uniform thickness;

(d) drying the coating at elevated temperature; and

(e) substantially immediately heat stabilizing the coating by raising the temperature thereof to the range of about 250 F. to 375 F. for a period of about five seconds to five minutes.

3. The process of claim 2 wherein said gel-free aqueous mixture is provided by slowly and uniformly adding said graphite particles, said wetting agent and said pH modifier to said silica sol over a period of time not less than about five minutes while subjecting said silica sol to mixing agitation.

4. The process of claim 2 including the step of applying opposed electrodes to said electrically-conductive sheet to form a radiant heating element.

5. A process for producing an electrically-conductive radiant heating element in sheet-like form comprising the steps of:

(a) slowly and uniformly adding to an alkali-stabilized colloidal silica sol and While subjecting the silica sol to agitation,

(l) electrolytic-grade graphite particles,

(2) wetting agent for said graphite particles, and

(3) suflicient pH modifier to prevent gelation of the resulting mixture during the processing steps hereinafter set forth;

(b) subjecting the resulting mixture to extended mechanical working over a period not less than about fifteen minutes so as to break up any agglomerates formed by the ingredients of said aqueous mixture without substantial pulverization of said graphite particles and to substantially-completely disperse said graphite particles uniformly throughout said colloidal 18 silica sol and coat the graphite particles individually and discretely with said colloidal silica sol, whereby a substantially homogenized dispersion results;

(c) applying the resulting highly-dispersed mixture uniformly to a nonconductive, dimensionally-stable asbestos-containing supporting web so as to form a coating of substantially-uniform thickness;

(d) drying the coating at elevated temperature;

(e) substantially immediately heat stabilizing the dried coating by raising the temperature thereof to the range of about 250 F. to 375 F. for a period of about five seconds to five minutes;

(f) aging the resulting coated web for a period no less than about six hours; and

(g) applying opposed electrodes to the aged web to form a radiant heating element.

6. A process for producing a resistively-stable, electrically-conductive sheet suitable for a radiant heating element comprising the steps of:

(a) slowly and uniformly adding to an alkali-stabilized colloidal silica sol while subjecting such silica sol to mixing agitation,

(l) electrolytic-grade graphite particles,

(2) an anionic Wetting agent for said graphite particles, and

(3) sufficient non-fugitive pH modifier to control pH of the resulting gel-free aqueous mixture in the range of about 8.1 to 9.4;

(b) subjecting said mixture to extended mechanical working including shear stresses for a period of at least fifteen minutes so as to break any agglomerates down to primary particle size without substantial pulverization of the particles, to substantially completely disperse said graphite particles uniformly throughout said colloidal silica sol and to coat the graphite particles individually and discretely with said colloidal silioa sol, whereby a substantially homogenized dispersion results;

(c) applying a uniformly-thick film of said dispersion to a nonconductive, dimensionally-stable, asbestoscontaining supporting web having a substantially uniform absorptivity;

(d) drying the film by exposing it to elevated temperatures in the range of about 250 F. to 450 F.; and

(e) substantially immediately stabilizing the dried film by raising the temperature thereof to about 250 F. to 375 F. for -a period of about five seconds to five minutes.

7. The process of claim 6 wherein said non-fugitive pH modifier comprises sodium silicate and said supporting web has a drop-test absorptivity, as defined herein, in the range of about 4 to 10 minutes.

8. The process of claim 6 wherein sufiicient water is present in the homogenized dispersion to produce a Zahn Cup viscosity of about 7 to 10 seconds (No. 4 cup).

9. A process for producing an electrically-conductive radiant heating element comprising the steps of:

(a) slowly and uniformly adding to an alkali-stabilized colloidal silica sol While subjecting such silica sol to mixing agitation,

(1) electrolytic-grade graphite particles,

(2) an anionic wetting agent for said graphite particles, and

(3) sufiicient non-fugitive pH modifier to control pH of the resulting gel-free aqueous mixture in the range of about 8.1 to 9.4;

(b) subjecting said mixture to extended mechanical working including shear stresses fora period of at least fifteen minutes so as to break any agglomerates down to primary particle size without substantial pulverization of the particles, to substantially completely disperse said graphite particles uniformly throughout said colloidal silica sol and to coat the graphite particles individually and discretely with said colloidal silica sol, whereby a substantially homogenized dispersion results;

(c) adding to said substantially homogenized dispersion a defoaming agent to minimize entrapped air therein;

(d) filtering the defoamed dispersion to remove residual agglomerates therein;

(e) applying a uniformly-thick film of said dispersion to a nonconductive, dimensionally-stable, asbestoscontaining supporting web having a substantially uniform absorptivity;

(f) drying the film by exposing it to elevated temperatures in the range of about 250 F. to 450 F.;

(g) substantially immediately stabilizing the dried film by raising the temperature thereof to about 250 F. 15

to 375 F. for a period of about five seconds to five minutes;

(h) aging the resulting coated web for a period no less than about six hours; and

(i) applying opposed electrodes to the aged web to form a radiant heating element. 7

References Cited UNITED STATES PATENTS 2,759,092 8/1956 Fortin 29-155.5 X 2,803,566 8/1957 Smith 117-216 2,952,761 9/1960 Smith 219-641 3,037,266 6/1962 Pfsister -n 29155.63 3,095,636 7/1963 Ruckelshaus 29--155.62

J. L. CLINE, Assistant Examiner.

JOHN F CAMPBELL, Primary Examiner. 

1. A RESISTIVELY-STABLE, ELECTRICALLY-CONDUCTIVE SHEET SUITABLE FOR RADIANT-HEATING ELEMENTS COMPRISING A NONCONDUCTIVE, DIMENSIONALLY-STABLE WEB SUPPORTING A CONDUCTIVE FILM, SAID FILM BEING PREPARED BY THE METHOD WHICH COMPRISES: (A) PROVIDING A GEL-FREE AQUEOUS MIXTURE COMPRISING AN ALKALI-STABILIZED COLLOIDAL SILICA SOL, ELECTROLYTICGRADE GRAPHITE PARTICLES, A WETTING AGENT FOR SADI GRAPHITE PARTICLES, AND SUFFICIENT PH MODIFIER TO PREVENT GELATION OF THE RESULTING MIXTURE DURING THE PROCESSING STEPS HEREINAFTER SET FORTH; (B) SUBJECTING SAID MIXTURE TO EXTENDED MECHANICAL WORKING INCLUDING SHEAR STRESSES SO AS TO BREAK UP AGGLOMERATES FORMED BY THE INGREDIENTS OF SAID AQUEOUS MIXTURE WITHOUT SUBSTANTUAL PULVERIZATION OF SAID GRAPHITE PARTICLES AND TO SUBSTANTIALLY-COMPLETELY DISPERSE SAID GRAPHITE PARTICLES UNIFORMLY THROUGHOUT SAID COLLOIDAL SILICA SOL AND COAT THE GRAPHITE PARTICLES INDIVIDUALLY AND DISCRETELY WITH SAID COLOIDAL SILICA SOL, WHEREBY A SUBSTANTIALLY HOMOGENIZED DISPERSION RESULTS; (C) APPLYING THE RESULTING DISPERSION UNIFORMLY TO SAID NONCONDUCTIVE, DIMENSIONALLY-STABLE SUPPORTING WEB SO AS TO FORM A COATING OF SUBSTANTIALLY-UNIFORM THICKNESS; (D) DRYING THE COATING AT ELEVATED TEMPERATURE; AND (E) SUBSTANTIALLY IMMEDIATELY HEAT STABILIZING THE COATING BY RAISING THE TEMPERATURE THEREOF TO THE RANGE OF ABOUT 250*F. TO 375*F. FOR A PERIOD OF ABOUT FIVE SECONDS TO FIVE MINUTES.
 2. A PROCESS FOR PRODUCING A RESISIVELY-STABLE, ELECTRICALLY-CONDUCTIVE SHEET COMPRISING THE STEPS OF: 